This invention relates to a method and apparatus for quickly disconnecting/connecting a mooring system for a turret moored drillship.
In the past, demand for a working platform around and upon the slope and continental shelf regions of the world has substantially increased with advances in technical capability. Examples of fixed station offshore facilities include supports for radar stations, light beacons, scientific and exploration laboratories, chemical plants, power generating plants, mining stations, etc. Principally, however, offshore facilities have been utilized by the oil and gas industry in connection with drilling, production, and/or distribution operations.
In shallow water applications, such as the nearshore portions of the Gulf of Mexico, fixed towers or platforms have been extensively utilized. In this regard, such offshore towers are typically fabricated onshore and transported in a generally horizontal posture to an offshore site upon a barge or buoyancy chambers within the tower legs. On site, the tower is pivoted into an upright posture and the base is positioned into secure engagement with the seabed. A platform deck is then fabricated upon the erected tower for conducting offshore operations. Such fixed platforms, although relatively economical, require considerable time to assemble and once in position are difficult to relocate.
One design which enhances the ability of a platform to be brought on station, operated, and then removed to a differing working site is known as a "jack-up platform". A jack-up platform typically comprises a barge or self-propelled deck operable to function in a conventional flotation capacity during transportation and in a working deck capacity on location. The deck carries a plurality of legs which are jacked downwardly from the deck into the seabed until the deck is raised a suitable working distance above a statistical storm wave height. Upon completion of desired operations, the deck is lowered down to the surface of the body of water and the legs are jacked back up to the deck. The platform is then towed or navigated to another working station where the process is repeated. Although jack-up platforms exhibit enhanced mobility, it will be recognized that jack-ups have depth limitation and for that reason are not always capable of being utilized in desired locations on the shelf.
In deep water applications, semi-submersibles or drillships are most often utilized. A drillship is constructed with a well opening, or moon pool, and the vessel is moored at a preselected site and/or dynamically positioned by thrusters. Drilling operations are conducted from the drillship through the moon pool. Of the presently known drillship designs it has been found that a turret moored drillship, such as disclosed in Richardson et al., U.S. Pat. No. 3,191,201 or Richardson U.S. Pat. No. 3,279,404 has been particularly effective. The foregoing Richardson et al and Richardson patents are of common assignment with the instant application and the disclosures thereof are incorporated herein by reference as though set forth at length. Briefly, however, a moon pool is fashioned through a central axis of the drillship and a turret is positioned within the moon pool. A set of eight mooring lines are then radially splayed from the turret and tensioned from winches mounted around the mooring turret. Once on station, with the mooring lines secure, the drillship is dynamically rotated about the turret through the utilization of retractable thrusters to maintain the vessel headed into prevailing wind and/or current. Station keeping for such turret moored drillships has been extremely reliable and accurate. Accordingly such drillships are the equipment of choice in deepwater applications.
Although previously known drillships have been advantageously utilized, certain situations occur which make it desirable to temporarily disconnect mooring lines from the turret moored vessel and then reconnect the lines and continue operations. In this connection, it has been found that a hurricane and/or typhoon in various shelf regions of the world may contain winds substantial enough to be potentially damaging to a working vessel. In addition, in polar regions of the world it is possible that a drillship may need to go off station in order to avoid collision from an iceberg. In such situations it has been found that although a drillship has the mobility to avoid environmental forces, rapidly releasing the drillship from its mooring lines can be troubling. Moreover, once an emergency environmental condition has passed it would be highly desirable to quickly reposition and reconnect the mooring lines to the drillship for continued on station operations.
In the past, systems have been at least theorized to disconnect and connect mooring lines to a drillship. In one design a mooring line is separated and fitted at each end with a sparbuoy carrying a remotely controlled disconnect element. Although this system has a degree of conceptual appeal, it is not believed that the operative integrity of the system is sufficient to reliably use the concept with a turret moored drillship.
Another previously known design includes a mooring line having an intermediate explosive link such that the mooring line can be selectively severed to quickly release the drillship. Although explosive links have been known in the past, it is not believed that the reliability of such systems is sufficient to warrant utilization in an environment where an error can produce consequential damages of tremendous magnitude. Moreover, explosive link systems can be difficult to reconnect during subsequent operations.
The difficulties suggested in the proceeding are not intended to be exhaustive, but rather are among many which may tend to reduce the effectiveness and user satisfaction with prior quick-disconnect/connect mooring systems. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that drillship mooring systems appearing in the past will admit to worthwhile improvement.